Permanent magnets have long been known and used at least since the first primitive compass was used in navigation. Today permanent materials are used extensively in electric and electronic assemblies. With the increased miniaturization and high efficiency of electrical equipment there has been an increasing demand for permanent magnet materials having higher coercive forces and energy products.
Typical permanent magnet material currently in use are alnico, hard ferrite and rare earth/cobalt magnets. The rare earth/cobalt magnets are capable of high magnetic properties, but are very expensive because they require the rare earth element, samarium, which is relatively scarce, and cobalt, the supplies of which have been uncertain in recent years.
In recent years new magnetic materials have been introduced containing iron, various rare earth elements and boron. Such magnets have been prepared from melt quenched ribbons and also by the technique of compacting and sintering which had been employed to produce samarium cobalt magnets.
U.S. Pat. No. 4,597,938 to Matsuura, Sagawa and Fujimura shows a process for the production of permanent magnet materials of the Iron-Boron-Rare Earth type in which the materials are prepared as a metallic powder compacted and sintered at high temperature while being held in a non-oxidizing or reducing atmosphere. U.S. Pat. No. 4,601,875 to Yamamota, Sagawa, Fujimura and Matsuura shows a similar process for producing magnetic materials but with the additional step that the sintered body is heat-treated at a temperature above 350.degree. C. still in a nonoxidizing atmosphere. The teachings of these references, both assigned to Sumitomo Special Metals Co., Ltd. are incorporated by reference herein.
In the compaction of the magnetic powder, however, certain problems arise. It is useful, for example, to apply a strong external magnetic field to orient the particles prior to compacting. Because an assembly of dry particles have limited mobility, however, it is often difficult to achieve a high degree of alignment. Secondly, and also because the dry particles have limited mobility, the compacted powder often has one or more voids or discontinuities where the compaction is not uniform and homogeneous.
In the preparation of hard ferrite magnetic materials, similar problems are overcome by dispersing the magnetic ferrite particles in water and wet pressing the resulting dispersion.